The invention relates generally to gas sensing apparatus, and more particularly to gas sensing apparatus utilizing the adsorbent properties of organic semiconductors.
There is pressing need for highly selective and sensitive detectors of gaseous constituents in a wide variety of industrial, medical, and military contexts. There is a need to detect and monitor contaminant gases in industrial purification processes; anesthetic gases in operating rooms; chemical warfare agents on the battlefields; and environmental pollutants in industrial or agricultural areas.
Many of the current approaches to contaminant gas detection lack sensitivity and selectivity. For example, the selectivity of individual piezoelectric sorption detectors of the type disclosed by King, Jr., U.S. Pat. Nos. 3,164,004 or by Frechette et al, 4,111,036, depends entirely on the selectivity of the adsorbent coatings on the piezoelectric crystals. The selectivity of these adsorbent materials are notoriously poor, e.g., Hirschfeld, "Providing Innovative System Monitoring and Reliability Assessment Through Microengineering", Energy Technology and Review, February 1984; and Edwards et al, "A Quartz Crystal Piezoelectric Device for Monitoring Organic Gaseous Pollutants", Analytica Chimica Acta, Vol. 117, pgs. 147-157 (1980). Selectivity problems can be overcome to a large extent by employing a plurality of sensors each having a different adsorbent coating. The responses of the multiple sensors are analyzed by factor analysis or least squares analysis to obtain information about the concentrations of contaminants in the gas being monitored.
Another approach to detecting contaminant gases utilizes their effects on the conductivities of certain organic semiconductors. The class of sensors based on this approach is referred to as chemoresistive gas sensors. A thin layer of organic semiconductor is disposed on a surface between two electrodes. The layer of organic semiconductor is exposed to the atmosphere containing the contaminant gases of interest. As the organic semiconductor adsorbs the contaminant gases, its conductivity changes, and the magnitude of the change is related to the concentration of contaminant gas. The following references provide representative disclosures of such gas detection systems: Colla et al, U.S. Pat. Nos. 4,142,400 and 4,236,307; Kolesar, 4,472,356; Bott et al, "A Highly Sensitive NO.sub.2 Sensor Based on Electrical Conductivity Changes in Phthalocyanine Films", Sensors and Actuators, Vol. 5, pgs. 43-53 (1984); and Hermans, "CO, CO.sub.2, CH.sub.4, and H.sub.2 O Sensing by Polymer Covered Interdigitating Electrode Structures", Sensors and Actuators, Vol. 5, pgs. 181-186 (1984). Like the piezoelectric-based sensors, the selectivity of chemoresistive sensors is limited by the lack of selectivity of the organic semiconductors for particular gaseous contaminants. This class of sensor is also limited by the extremely low conductivity of the layers of organic semiconductor, which is on the order of 10.sup.-10 -10.sup.-8 ohms.sup.-1. The low conductivity means that the changes introduced by the adsorbed contaminants will be correspondingly low and difficult to detect; hence, the sensitivity of the system is low.